In the manufacture of most integrated circuits, particularly very large scale integrated circuits such as arrays of DRAMs (Dynamic Random Access Memory) or PROMs (Programmable Read Only Memory), numerous identical circuits are placed on the same wafer of semiconductor material. This is accomplished by creating various elements of the circuits in multiple layers. Each element is ordinarily produced by a lithographic process wherein an image of one or a few of the individual circuit elements is projected onto the wafer at a time with the wafer being secured to a movable stage. After a first projection is made, the stage is stepped to a new position where the same image is projected again. This process is typically repeated until all of the desired elements of that particular layer are imprinted on the wafer. That layer is then treated to form the desired elements and then subsequent layers of elements are added over the first until all of the elements of the circuit are produced.
Because of the high circuit density on the wafer and the need to make uniform and repeatable connections to the circuits thereon, it is very important that the masking material which is used to define such elements and connections be patterned in a manner which is repeatable and accurate. Typically, photoresist is used as a masking layer and is exposed to illumination through a mask which defines desired circuit elements and connections. Various processing parameters can affect the accuracy with which the photoresist can be patterned. For example, if a mask is misaligned, images will not be accurately reproduced within the photoresist. Similarly, if the illumination exposure dosage is too great or not enough, the photoresist will be over-exposed or underexposed respectively thereby rendering inaccurate images. Needless to say, maintaining accurate control over parameters such as illumination exposure dosage, light scattering, focus, and/or photoresist developing parameters is very important.
This invention arose out of concerns associated with providing methodologies that facilitate monitoring and control of semiconductor processing conditions.